I'm surprised, I'd have advised the same as daf. One thing I'll say is there is usually a real fix, but SOMETIMES, and rarely, there isn't. Once in a blue moon, I find the only way to fix problems like this (if my mesh is indeed TOTALLY correct, no unwelded verts, no ngons, etc.) is to convert the object to Editable mesh, and then convert back to editable poly.
Make a copy of your scene in case you wish to avoid collapsing your stack, which is unavoidable if you need this "reset". If this doesn't help, then you know there REALLY is a flaw in your mesh.
If you post the mesh, one of us will find it if you give up.
